When exhaust gas cools down in an internal combustion engine of a vehicle, such as a diesel engine, the bed temperature of an exhaust gas purifying catalyst, which is arranged in an exhaust passage, drops. This hampers the function of the catalyst to purify exhaust gas. To solve this problem, an exhaust emission purification controller described in Patent Document 1 performs bed temperature control, in which unburned fuel is added to exhaust gas by opening a fuel adding valve, which is provided in an exhaust passage, when the temperature of the exhaust gas decreases. In other words, the controller forcibly increases the temperature of the exhaust gas through such bed temperature control.
The controller of the aforementioned document stores a learned value of the addition amount of the unburned fuel for each one of various types of operating ranges of the internal combustion engine in order to correct the difference between the bed temperature and a target catalyst bed temperature at the time when the bed temperature control is carried out. The controller reflects the learned value in the addition amount of the fuel.
The learned value is obtained under such conditions that the bed temperature control is currently carried out and the target bed temperature is greater than or equal to a predetermined value. The learned value is determined from the difference between the actual fuel supply amount and the estimated fuel supply amount. The actual fuel supply amount is the amount of the fuel supplied to the exhaust gas purifying catalyst. The estimated fuel supply amount is the amount of the fuel that has actually contributed to rise of the catalyst bed temperature.
The learned value is also learned when the catalyst bed temperature. That is, the learned value includes a component for correcting change over time of the fuel adding valve and component for correcting compensation of the catalyst bed temperature. The component for correcting change over time of the fuel adding valve corrects an error of the fuel adding valve caused by change over time. The component for correcting compensation of the catalyst bed temperature corrects an error of the bed temperature with respect to the target catalyst bed temperature caused by the heat capacity of the exhaust gas purifying catalyst at the time when the catalyst bed temperature changes.
However, the exhaust emission purification controller of the aforementioned document does not separately learn the component for correcting change over time and the component for correcting compensation of the catalyst bed temperature. Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-172185